Thermal reactors are designed to operate over a considerable period of time and thus must contain an excess of fissionable material over that amount just necessary to sustain a chain reaction. Such an excess reactivity must be controlled, typically by the use of shim rods or by the inclusion of a burnable poison in the core. However, both of these alternatives have the undesirable result that a considerable number of neutrons are lost by capture in a non-productive absorber. When a thermal reactor is used as a breeder reactor as well, the loss of such neutrons in a non-productive manner additionally affects the ability of the reactor to breed.
In U.S. Pat. No. 3,141,827 (Iskenderian), a breeder reactor design is disclosed which has good neutron economy. This is achieved by the use of depleted fuel elements to obtain shim control in the reactor. The depleted fuel elements absorb neutrons to produce additional fissionable material. The depleted fuel elements are interspersed among slightly enriched elements. In U.S. Pat. No. 3,122,484 (Iskenderian), the use of fuel elements containing a predominant amount of fertile material interspersed with low enrichment fuel elements is disclosed. The fertile fuel elements are covered with a burnable poison. As the enriched fuel is depleted, the fertile material is converted to fissionable material to help compensate for the loss of reactivity in the enriched fuel elements.
In U.S. Pat. No. 3,351,532 (Raab, Jr. et al), a seed-blanket converter-recycle breeder reactor is disclosed in which the fissile and fertile fuel concentrations and their positions within the core are chosen to provide both an extended reactivity lifetime and an increase in fissile fuel inventory. The reactor has a plurality of seed regions or islands in the reactor core surrounded by blanket regions.
A composite reactor which reduces the BOL critical mass of fast systems by using high reactivity thermal regions is disclosed in U.S. Pat. No. 3,093,563 (Menke). The reactor core includes a fast central region where little moderation takes place and a surrounding annular region where greater modulation takes place.
Various designs for fast nuclear reactors have also been disclosed in which zoning of fuel regions have been proposed. For example, in U.S. Pat. No. 3,338,790 (Ackroid et al), a fast reactor is disclosed in which a fast fuel region is surrounded by a blanket area with a coolant zone located therebetween. In U.S. Pat. No. 2,992,982 (Avery), a scheme is disclosed for coupling a small thermal reactor region to a fast reactor to enhance the safety of the fast system without degrading significantly the breeding characteristics of the fast reactor. According to the design disclosed, there is an enriched fuel core surrounded by an inner blanket annulus containing natural uranium and the sodium coolant. Outside of the inner blanket is an annulus of beryllium which in turn is surrounded by an outer blanket of depleted uranium. In U.S. Pat. No. 3,297,539 (Beckurts et al), a fast reactor design is disclosed in which a fast core made of fissile material is surrounded by a blanket made of natural uranium. The natural uranium blanket is in turn surrounded by a reflector made of graphite.